Aldol Additions of Titanium and Boron Enolates of Achiral and Chiral δ‐Lactones to Achiral Model Aldehydes: Simple and Induced Diastereoselectivities

We studied the steric course of aldol additions of CpTiCl 2 (novel) or Bu 2 B enolates of the unsubstituted δ‐lactone and of the four monomethylated δ‐lactones to isobutyraldehyde, crotonaldehyde, and para ‐bromobenzaldehyde. The titanium enolates reacted syn ‐selectively with >95:5 ds in most cases. The boron enolates reacted anti ‐selectively without exception ( ds = 98:2 to 92:8). These selectivities paired with a preferred trans ‐orientation of the α‐hydroxyalkyl substituents relative to the lactone's β‐ or γ‐methyl group and with a preferred cis ‐orientation relative to the lactone's δ‐methyl group. Our preparation of γ‐methyl‐δ‐lactone ( 20 ) features a tandem glycol cleavage/lactol → lactone conversion with cat. TEMPO/stoichiom. PhI(OAc) 2 , which we believe is novel.